The preparation of conjugates by forming a covalent linkage between the water soluble polymer and the therapeutic protein can be carried out by a variety of chemical methods. PEGylation of polypeptide drugs protects them in circulation and improves their pharmacodynamic and pharmacokinetic profiles (Harris and Chess, Nat Rev Drug Discov. 2003; 2:214-21). The PEGylation process attaches repeating units of ethylene glycol (polyethylene glycol (PEG)) to a polypeptide drug. PEG molecules have a large hydrodynamic volume (5-10 times the size of globular proteins), are highly water soluble and hydrated, non-toxic, non-immunogenic and rapidly cleared from the body. PEGylation of molecules can lead to increased resistance of drugs to enzymatic degradation, increased half-life in vivo, reduced dosing frequency, decreased immunogenicity, increased physical and thermal stability, increased solubility, increased liquid stability, and reduced aggregation. The first PEGylated drugs were approved by the FDA in the early 1990s. Since then, the FDA has approved several PEGylated drugs for oral, injectable, and topical administration.
Polysialic acid (PSA), also referred to as colominic acid (CA), is a naturally occurring polysaccharide. It is a homopolymer of N-acetylneuraminic acid with α(2→8) ketosidic linkage and contains vicinal diol groups at its non-reducing end. It is negatively charged and a natural constituent of the human body. It can easily be produced from bacteria in large quantities and with pre-determined physical characteristics (U.S. Pat. No. 5,846,951). Because the bacterially-produced PSA is chemically and immunologically identical to PSA produced in the human body, bacterial PSA is non-immunogenic, even when coupled to proteins. Unlike some polymers, PSA acid is biodegradable. Covalent coupling of colominic acid to catalase and asparaginase has been shown to increase enzyme stability in the presence of proteolytic enzymes or blood plasma. Comparative studies in vivo with polysialylated and unmodified asparaginase revealed that polysialylation increased the half-life of the enzyme (Fernandes and Gregoriadis, Int J Pharm. 2001; 217:215-24).
Coupling of PEG-derivatives to peptides or proteins is reviewed by Roberts et al. (Adv Drug Deliv Rev 2002; 54:459-76). One approach for coupling water soluble polymers to therapeutic proteins is the conjugation of the polymers via the carbohydrate moieties of the protein. Vicinal hydroxyl (OH) groups of carbohydrates in proteins can be easily oxidized with sodium periodate (NaIO4) to form active aldehyde groups (Rothfus et Smith, J Biol Chem 1963; 238:1402-10; van Lenten et Ashwell, J Biol Chem 1971; 246:1889-94). Subsequently the polymer can be coupled to the aldehyde groups of the carbohydrate by use of reagents containing, for example, an active hydrazide group (Wilchek M and Bayer E A, Methods Enzymol 1987; 138:429-42). A more recent technology is the use of reagents containing aminooxy groups which react with aldehydes to form oxime linkages (WO 96/40662, WO2008/025856).
Additional examples describing conjugation of a water soluble polymer to a therapeutic protein are described in WO 06/071801 which teaches the oxidation of carbohydrate moieties in Von Willebrand factor and subsequent coupling to PEG using hydrazide chemistry; US Publication No. 2009/0076237 which teaches the oxidation of rFVIII and subsequent coupling to PEG and other water soluble polymers (e.g. PSA, HES, dextran) using hydrazide chemistry; WO 2008/025856 which teaches oxidation of different coagulation factors, e.g. rFIX, FVIII and FVIIa and subsequent coupling to e.g., PEG, using aminooxy chemistry by forming an oxime linkage; and U.S. Pat. No. 5,621,039 which teaches the oxidation of FIX and subsequent coupling to PEG using hydrazide chemistry.
Recently, an improved method was described comprising mild periodate oxidation of sialic acids to generate aldehydes followed by reaction with an aminooxy group containing reagent in the presence of catalytic amounts of aniline (Dirksen A., and Dawson P E, Bioconjugate Chem. 2008; 19, 2543-8; and Zeng Y et al., Nature Methods 2009; 6:207-9). The aniline catalysis dramatically accelerates the oxime ligation, allowing the use of very low concentrations of the reagent. The use of nucelophilic catalysts are also described in Dirksen, A., et al., J Am Chem Soc., 128:15602-3 (2006); Dirksen, A., et al., Angew chem. Int Ed., 45:7581-4 (2006); Kohler, J. J., ChemBioChem., 10:2147-50 (2009); Giuseppone, N., et al., J Am Chem Soc., 127:5528-39 (2005); and Thygesen, M. B., et al., J Org Chem., 75:1752-5 (2010).
Although aniline catalysis can accelerate the oxime ligation allowing short reaction times and the use of low concentrations of the aminooxy reagent, aniline has toxic properties that must be considered when, for example, the conjugated therapeutic protein to form the basis of a pharmaceutical. For example, aniline has been shown to induce methemoglobinemia (Harrison, J. H., and Jollow, D. J., Molecular Pharmacology, 32(3) 423-431, 1987). Long-term dietary treatment of rats has been shown to induce tumors in the spleen (Goodman, D G., et al., J Natl Cancer Inst., 73(1):265-73, 1984). In vitro studies have also shown that aniline has the potential to induce chromosome mutations and has the potentially genotoxic activity (Bombhard E. M. et Herbold B, Critical Reviews in Toxicology 35,783-835, 2005).
Considering the potentially dangerous properties of aniline and notwithstanding the methods available of conjugating water soluble polymers to therapeutic proteins, there remains a need to develop materials and methods for conjugating water soluble polymers to proteins that improves the protein's pharmacodynamic and/or pharmacokinetic properties while minimizing the costs associated with the various reagents and minimizing the health risks to the patient recipient.